The Dawn spacecraft finds signs of water on the protoplanet Vesta and links meteorites on Earth to the massive asteroid. Scientists continue to search it for clues to Earth's development.

An up-close look at the protoplanet Vesta taken by the Dawn spacecraft reveals signs of water on this oversized asteroid in the middle of the solar system, scientists reported Thursday in the journal Science.

Vesta floats in the middle of the asteroid belt that fills the gap between the orbits of Mars and Jupiter. That doughnut of rocky rubble might have coalesced into a whole planet if Jupiter's gravity hadn't gotten in the way. Instead, Vesta's growth was stunted at the protoplanet stage.

Because Earth must have gone through the same developmental stages — and because Vesta's rocks haven't been totally churned up by volcanic processes — the giant asteroid provides valuable clues as to what our planet's early history might have looked like. Scientists have studied meteors strongly believed to be fragments of Vesta that fell to Earth, but haven't had conclusive proof that the two were linked — until now.

Researchers scanning Vesta's surface with Dawn's gamma ray and neutron detector were able to definitively link the protoplanet to those Earth meteorites by examining the ratios of iron to silicon and to oxygen. The detector charts the abundance of elements like hydrogen, oxygen and silicon by analyzing the distribution of neutrons and gamma radiation emitted when cosmic rays smash into the surface.

Cameras and spectrometers on Earth have already taught planetary scientists much about Vesta's surface. But those neutron and gamma ray signals can't be picked up by detectors on Earth.

"That's the most important result returned by the entire mission," said Richard Binzel, a planetary scientist at MIT who was not involved in the studies. "That instrument can only be successful when it's up close and personal. It's a measure that could only be made right there at Vesta."

Even as it confirmed a long-held theory, the detector turned up a surprise find: Parts of Vesta's surface contained unusually high levels of hydrogen. Though researchers can't say what molecular form it's in, the amount of hydrogen picked up in parts of the surface would be enough to support 400 parts per million of water, a likely hydrogen-containing compound. (By comparison, rocks brought back by the Apollo missions contained 50 parts per million.)

The find caught researchers off guard. If water is anywhere, it typically clings to the polar regions, not at the much-warmer equator.

On Vesta, "the picture you have is almost the reverse," said Thomas Prettyman, an Albuquerque-based scientist at the Planetary Science Institute in Tucson, who led one of two studies published in Science.

Prettyman and his colleagues wondered whether cosmic rays had deposited protons — which are basically just hydrogen ions — onto Vesta's surface. But they concluded there weren't enough cosmic rays hitting Vesta to account for all of the hydrogen.

The second study provided a potential answer. A team led by Brett Denevi, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., scrutinized unusual pits marring Vesta's terrain. They weren't craters, because they lacked the distinctive raised rim caused by impacts on the surface. In fact, Denevi said, they looked very much like Martian pits thought to be caused by water vapor escaping the Red Planet's surface.

"The expectation was a dry body, but that turned out not to be true," Denevi said.

The scientists think water-rich asteroids once pummeled Vesta, layering hydrated minerals onto the protoplanet's surface. Later, other asteroids smashing into the surface heated up the rocks and caused the water trapped inside to evaporate, leaving the telltale pits behind.

"It all fits," Binzel said. "It's a very conclusive case."

Dawn, which was built at the Jet Propulsion Laboratory in La Cañada-Flintridge, left Vesta's orbit Sept. 4 and is now heading to Ceres, an icy dwarf planet in the asteroid belt.